Several techniques and methods have been used to measure subsurface electrical properties in order to obtain geological information about underground structures. There are three closely related electrical geophysical techniques (EGT); electrical resistivity tomography (ERT), electrical impedance tomography (EIT) and controlled source electromagnetics (CSEM). These techniques seek to determine subsurface electrical properties and operate at audio or sub audio frequencies.
Measurements are made by inducing current flow through a pair of electrodes and simultaneously monitoring induced voltages in additional pairs of electrodes and their connecting wires. The electrodes are formed from metal or graphite placed either directly in the ground or placed inside a porous container containing a salt solution which in turn is placed in the ground. Because of the simple, robust nature of the electrodes, EGT systems are amenable to either characterization mode where single surveys are made to locate subsurface features, or as a monitoring tool where the electrodes are permanently placed and the technique is used to monitor the changes in the subsurface over time.
The primary differences between the techniques are in the interpretation methods applied to the resulting data. The ERT technique assumes that the electrical potential can be accurately approximated using a steady-state approximation based on Laplace's equation. The EIT technique assumes that the intrinsic properties are frequency dependent but that the electrical potential can still be approximated using a complex Laplace's equation. The CSEM method requires a full solution to Maxwell's electromagnetic equations.
These methods provide tools for imaging subsurface electrical resistivity distributions. Because of this imaging capability, they can also be used for inferring fluid flow and transport. They can be applied to a range of depths, well spacings, and reservoir types, and could be used to monitor oil field stimulation applications, such as water and steam flooding. Anything that changes the electrical resistivity of the subsurface area can be monitored. Because electric current flows through the pore fluid of an underground reservoir, electrical methods are especially sensitive to pore fluid content and have been applied to mineral exploration, environmental, oil field and industrial projects.
U.S. Pat. No. 7,805,249 purports to disclose a method for performing a controlled source electromagnetic survey of a subterranean region. U.S. Pat. No. 7,773,457 purports to disclose a wireless exploration seismic system that acquires seismic data using a data acquisition module that collects seismic data and forwards the data to a central recording and control system. U.S. Pat. No. 7,386,402 purports to disclose an apparatus for transmitting and detecting geophysical data using reconfigurable control units.